It has long been known to produce from thermoplastic polymeric material a double wall pipe, the outer wall being corrugated generally transversely of the pipe, the inner wall being smooth and uncorrugated, the inner wall and outer wall being fused together at the inner crests of the corrugations of the outer wall. Pipes of this type are produced by simultaneously extruding both an inner and outer tube; while still in the plastic state, inflating the outer tube against a surrounding mold which moves with the extruded outer tube, so as to corrugate the outer tube, and then urging the inner tube against the inner crests of the corrugations of the outer tube, again while the material of both tubes is still in a plastic state. Typical methods and apparatus of the prior art are disclosed in the following U.S. Pat Nos.:
3,280,430--Antrobus PA0 3,538,209--Hegler PA0 3,677,676--Hegler PA0 3,743,457--Cini PA0 3,994,646--Hauck PA0 3,996,323--Hegler et al. PA0 4,305,703--Lupke et al.
When the polymeric material employed has been polyethylene, the methods and apparatus employed by prior-art workers have been quite successful, and such double wall pipe of polyethylene has become a common article of commerce. However, though extensive work appears to have been done, little commercial success has heretofore been achieved in attempting to extrude and form such double wall pipe from polyvinyl chloride, despite the fact that polyvinyl chloride is well known to have the superior characteristics which would make it possible for such double wall pipe to have the higher engineering properties required, e.g., for use as sewer pipe. In at least most cases, polyvinyl chloride pipe produced according to the prior art appears to have been characterized by either excessive brittleness, inadequate fusion between the two walls, or both.
Reasons for difficulties encountered in the prior art are not completely understood. One important factor is that polyethylene, with which prior workers have succeeded, loses heat less quickly than does polyvinyl chloride, so that prior-art workers using polyethylene have been able to bring the inner tube into engagement with the crests of the corrugated outer tube while both tubes are still at temperatures adequate for good fusion, but have not been able to do so with polyvinyl chloride. It is also likely that the flow patterns to which the plastic material has been subjected in order to achieve fusion of the two walls have resulted in constrained flow shear which, when a polymer such as polyvinyl chloride is employed, causes the product to be excessively brittle. Regardless of the reasons for the difficulties encountered in the prior art, there has been a continuing demand for double wall pipe of the type referred to made from polyvinyl chloride and having the engineering characteristics required when the pipe is to be used as sewer pipe or for like purposes.